The present invention relates to a steering control apparatus for a vehicle, and particularly relates to a steering control apparatus capable of restraining deflection of the vehicle, when the vehicle is turning, with an anti-skid control or traction control being performed on a μ-split road surface.
In Japanese Patent Laid-open Publication No. 2004-532153, which corresponds to US Publication No. 20050189163A1, with respect to a stability compensation system for stabilizing a vehicle, it is proposed to provide the system which is capable of stabilizing the vehicle by adjusting a steering wheel, when a split μ (may be called as a μ-split) braking operation is performed to cause unbalanced braking torque, thereby to deteriorate the vehicle stability. It is described in the US Publication that there is provided a vehicle stability compensation system which is arranged to adjust dynamically the self-centering position and the steering feel of the steering system during split μ braking operation, the adjustment being based on at least one operational variable representing a corrective steer angle for the vehicle which is added to the main EAS assistance torque via a driver feedback controller whereby to maintain the vehicle stable and controllable.
Then, it is described that one possible operational variable representing a corrective steer angle is the braking yaw moment. This can be established, for example by generating and subtracting from each other, estimates of the brake pressures at the front left and front right wheels, multiplying the difference by a constant to give the difference in brake forces for the front wheels, and dividing the result by the track width of the vehicle. The braking yaw moment is multiplied by a gain to give the corrective steer angle. A second possible operational variable representing a corrective steer angle is yaw oscillation. This can be established, for example, by inverting a yaw rate signal, multiplying this by a gain and using the result as a feedback signal providing yaw oscillation correction. And, it is described that a vehicle model is driven by inputs of steering angle (at the road wheels), yaw moment disturbance input and vehicle speed.
Furthermore, Japanese Patent No. 2676726 discloses a pressure estimation in case of an anti-skid control, as will be described later, and Japanese Patent No. 3443951 discloses a pressure estimation in case of an traction control, as will be described later.
However, according to the invention as disclosed in Japanese Patent Laid-open Publication No. 2004-532153, it relates to the stability compensation system for stabilizing the vehicle, when the anti-skid control is performed on the so-called μ-split road surface, with different coefficients of friction being provided at the right and left front wheels, and the control prevails completely, as long as the vehicle driver is following the control. Then, it is so constituted that the corrective steer angle is obtained on the basis of a difference in braking pressure between the right and left front wheels, or an error between an estimated yaw rate (may be called as desired yaw rate) based on a vehicle speed and steering angle, and an actual yaw rate, and that an assist torque for an electric assist steering system (may be called as electric power steering system) is controlled on the basis of the corrective steer angle.
With respect to the electric power steering system, the steering wheel and steered wheels are mechanically connected with each other, to provide one to one relationship through a steering gear ratio. According to the system with the control prevailing completely as disclosed in the aforementioned Publication No. 2004-532153, it is possible to calculate the desired yaw rate based on the vehicle speed and steering angle, even in the case where the μ-split road surface exists in the turning course of the vehicle. However, according to such an assist system that the steering maneuver of the vehicle driver prevails, and that is aimed to assist the maneuver of the vehicle driver with the steering torque, it is difficult to be determined whether the steering angle of the steering wheel is resulted from the intention of the vehicle driver, i.e., the maneuver of the vehicle driver, or the one resulted from the control. Therefore, it may be caused that the desired yaw rate based on the vehicle speed and steering angle can not be determined.